1. Technical Field
The disclosure relates to a reversible thermal thickening grease, especially for microelectronic packages, a method of making the reversible thermal thickening grease, and a microelectronic package containing the reversible thermal thickening grease. In particular, the disclosure relates to a reversible thermal thickening grease for microelectronic packages, in which the grease contains filler particles and a binder, in which one or more segments of at least one polymer may be attached to surfaces of the particles and/or included in the grease without attachment, such that the fluidity of the grease minimizes grease pump-out that leads to air in the chip-heat sink interface.
2. Discussion of the Background
Thermal greases have been used extensively for microelectronic flip-chip packages as thermal interface materials (TIMs) with a two-fold objective:
(i) to improve conduction of heat away from the chip; and
(ii) to provide a compliant interface that will accommodate thermal expansion mismatches between the various materials in the package.
A flip-chip package may conventionally include a direct electrical connection of a face down (“flipped”) electronic circuit chip onto a substrate having electrical conductors, such as a ceramic substrate, circuit board, or carrier using conductive solder bumps formed in a ball grid array on bond pads of the chip. A TIM is disposed on an upper side of the electronic circuit chip, in which a substantially flat thermally conductive lid is disposed over the chip and in thermal contact with the interface material. In particular, the TIM is located in the gap between the chip and the lid for transmitting heat generated from the chip to the lid.
Thermal grease TIMs, also referred to as thermal pastes consist of a metal or other highly conductive (thermal) solid fillers in an organic binder. They are specifically formulated to exhibit fluid responses under external stresses to minimize contact resistance and accommodate system stresses. As the flip-chip package continues to evolve, in terms of substrates used (ceramic vs. organic), chip operating power etc., these greases are being subjected to larger thermal and mechanical stresses that exacerbate their degradation over time.
In particular, thermal greases are subject to being pumped out of the gap with the repetitive expansion and contraction of the package that occurs with thermal excursions. During its active state, the chip temperature increases leading to an overall rise in the package temperature and thermal expansion of various materials in the package with a net confinement pressure exerted on the grease. Once this confinement pressure is greater than the grease yield stress, the grease is forced to flow to regions of lower pressure i.e., the edges. This flow is exacerbated by a decrease in the binder viscosity with increased temperature. Depending on the relative ratio of capillary to viscous forces, some of the binder may be drained from the interstitial pores during this process. When the chip and package cools during chip idle or off state, the package contracts and there is a negative pressure in the gap that results in the grease flow back into the gap. However, as the temperature decreases, the binder viscosity increases such that the amount of grease that flows back into the gap is less than that which flowed out. Air occupies areas devoid of grease and may also displace the binder in the grease. With repeated heating and cooling cycles, most of the grease can be pumped out of the chip-heat sink interface. This effect is commonly referred to as grease pump-out which negatively impacts heat removal from the package.
Greases have been used successfully in ceramic packages with few major degradation concerns due to the low thermal expansion mismatch between the ceramic substrate and other materials in the package. However, as the industry moves towards organic packages, in an effort to reduce the package cost, a stable TIM is needed to withstand the large compressive and shear forces encountered in these packages. Currently, there are no known methods or strategies of minimizing grease pump-out while retaining the grease fluidity with repetitive thermal stresses. However, new strategies exist such as the formulation of greases that can be crosslinked at high temperature into permanent gels after compression to the desired bondline during package assembly.
Though these gels overcome the problems associated with grease pump-out, the absence of fluidity in the system negatively impacts stress accommodation by the TIM. It is commonly observed that gel TIMs fracture with tensile loading during package contraction in the cooling phase of the thermal cycle. In view of the foregoing, there remains a need for a TIM such as a grease with an improved overall performance in microelectronic packages, in which the problems of grease pump-out and subsequent air proliferation are reduced or eliminated.